1. Field of the Invention
This invention relates to a track control system for tracking a laser light beam along the center of an optical recording medium, and more particularly to a tracking control method and apparatus that is adaptive to an optical recording medium having wobbled groove signal tracks. Also, this invention is directed to an optical recording medium suitable for the tracking control method.
2. Description of the Prior Art
Recently, optical recording media have been prevalent as recording media for recording information such as video and audio data. Such optical recording media include a read-only type disc such as CD-ROM, DVD-ROM, etc.; a write-once-read-many (WORM) type disc such as CD-R, DVD-R, etc.; and a rewritable type disc such as CD-RW, DVD-RAM, etc.
As shown in FIG. 1, the read-only type disc includes spiral-shaped tracks 2 extending from the inner circumference into the outer circumference thereof. Information pit trains 4 representing recorded information are defined on the track 2. The information pit trains 4 are usually arranged in a line along a reference line and distances between adjacent tracks are constant.
Typically, a one-beam or three beam system tracking control is used to reproduce the information recorded on the pit trains 4 on the tracks of the read-only type disc. The one-beam or three-beam system tracking control allows a light beam to trace the track depending upon a symmetry of the information pit trains in the width direction of the tracks.
On the other hand, in the WORM type disc and the rewritable optical disc, guiding grooves for the tracking control are provided instead of the pit trains. An example of such a disc is a CD-R type disc shown in FIG. 2.
The CD-R in FIG. 2 has land and groove signal tracks 10 and 12 that are defined in parallel in a spiral shape. Each groove signal track 12 has a wobbled area in which both sides thereof are wobbled in the same phase; while each land signal tracks 10 has a wobbled area in which both sides thereof are wobbled in a different phase. Information is recorded on the same-phase wobbled area. In other words, information is recorded only on the groove signal tracks 12. In a disc having the land and groove tracks 10 and 12 as described above, the tracking control is easy because the boundaries between adjacent tracks 10 and 12 are distinguished. However, the recording capacity is reduced because an information is recorded only on the groove signal tracks 12.
To increase the recording capacity of the disc having the land and groove signal tracks, a so-called `land/groove recording system disc` has been developed, wherein information can be recorded on both the land and groove signal tracks like a DVD-RAM shown in FIG. 3.
The DVD-RAM of FIG. 3 has land and groove signal tracks 10 and 12 that are divided into a number of sectors defining the size of an information to be recorded. Each sector includes an identification information area, hereinafter referred simply to as "ID area", in which an address information and the like is recorded in prepit trains 14, and a recording area in which information can be recorded on both land and groove tracks 10 and 12.
Each prepit train 14 of the ID area is divided into a first prepit train 14a for the land track and a second prepit train 14b for the groove track. These first and second prepit trains 14a and 14b are positioned at an extending line of the land and groove boundary side, respectively. Since the first and second prepit trains 14a and 14b are positioned as mentioned above, an optical pickup reads all the first and second prepit trains 14a and 14b positioned at the extending line of the boundary side of a track being currently traced when a laser light beam traces the land track 10 as well as the groove track 12.
Two identification information signals for two prepit trains 14a and 14b read in this manner are processed employing only one of the prepit trains, for example, only the first prepit train 14a when a light beam is positioned at the land track 10. Accordingly, an information is recorded on both the land and groove tracks 10 and 12.
To accurately track an optical disc, it is necessary to have a tracking control apparatus for controlling a position of light beam in such a manner that the light beam is traced along the center lines of the land and groove tracks 10 and 12. A tracking control apparatus of push-pull system is usually used as such a tracking control apparatus. The push-pull system tracking control apparatus performs a tracking on a basis of a tracking error signal Te produced by subtracting two photo detecting signals from a two-divisional photo detector 20 as shown in FIG. 5.
However, the push-pull system tracking control apparatus must invert the polarity of a tracking error signal Te for one of the land and groove tracks 10 and 12 when it is used for an optical disc of land/groove recording system as mentioned earlier. This is caused by a fact that the tracking error signal Te has positive(+) and negative(-) amplitudes as shown in FIG. 4 as the light beam is moved in a radial direction.
Referring to FIG. 4, the tracking error signal Te has a positive(+) amplitude when the light beam is positioned between the center line of the land track 10 and the center line of a groove track 12 when moving towards the inner circumference; and has a negative(-) amplitude when the light beam is positioned between the center line of the land track 10 and the center line of a groove track 12 when moving towards the outer circumference. Accordingly, the tracking control apparatus must respond to the tracking error signal Te depending upon whether a light beam is traced along the land track 10 or the groove track 12.
More specifically, if a light beam is traced along the land track 10 and the tracking error signal Te has a negative(-) polarity, then the tracking control apparatus must move the light beam toward the inner circumference. Otherwise, if a light beam is traced along the groove track 12 and the tracking error signal Te has a negative(-) polarity, then the tracking control apparatus must move the light beam toward the outer circumference. As a result, the push-pull system tracking control apparatus must identify the land and groove tracks and control the polarity of the tracking error signal based on the identified result when it is used for an optical disc of land/groove recording system.
For instance, as shown in FIG. 5, a conventional push-pull system tracking control apparatus used for an optical disc of land/groove recording system includes a subtractor 22 and a low pass filter(LPF) 24 that are connected, in cascade, to a two-divisional photo detector 20. The two-divisional photo detector 20 converts the light beam reflected by the land or groove track 10 or 12 into an electrical signal to generate first and second photo detecting signals for representing a light distribution state. The subtractor 22 subtracts the first and second photo detecting signals from the two-divisional photo detector 20 to produce a tracking error signal Te. As shown in FIG. 4, the tracking error signal Te has a waveform that changes polarities as the light beam is moved in the radial direction of the optical disc. The LPF 24 eliminates high frequency noise signals included in the tracking error signal Te.
The push-pull system tracking control apparatus further includes a buffer 26 and an inverter 28 connected, in parallel, to the LPF 24, a control switch 30 for selecting any one of output signals of the buffer 26 and the inverter 28, and a tracking controller 32 and a tracking actuator 34 connected, in series, to the control switch 30. The buffer 26 passes the tracking error signal Te from the LPF 24 and the inverter 28 inverts the tracking error signal Te from the LPF 24, and both signals Te and Te' are applied to the control switch 30.
The control switch 30 delivers either the tracking error signal Te or the inverted tracking error signal Te' to the tracking controller 32 in accordance with a logical value of a land/groove(L/G) identification signal, that is, in accordance with whether a light beam is being currently traced along the land track 10 or the groove track 12.
Then, the tracking controller 32 applies a tracking drive signal to the tracking actuator 34 in accordance with the tracking error signal Te or the inverted tracking error signal Te' from the control switch 30. The tracking actuator 34 responding to the tracking drive signal moves an objective lens (not shown) in the radial direction of an optical disc, thereby tracing the light beam along the center line of the land or groove track 10 or 12. The push-pull system tracking control apparatus having the above-mentioned configuration must identify whether the light beam is being traced along the land track 10 or the groove track 12, and must selectively invert the tracking error signal in accordance with the land/groove identification result.
With the optical disc as shown in FIG. 3, the land/groove identification becomes possible with the aid of the prepit trains 14 in the ID area. However, the prepit trains 14 may be defective or contaminated with dust making it impossible to accurately detect the prepit trains, which in turn make it impossible to identify land or groove tracks. This results in the push-pull system tracking control apparatus of FIG. 5 being prevented from tracing the light beam along the center lines of the land and groove tracks. In other words, conventional tracking control method applied to the disc of land/groove recording system requires an accurate land/groove track identification for stable tracking control.